1. Technical Field of the Invention
The present invention relates generally to a glow plug energization controlling apparatus which may be employed in automotive vehicle to control energization of a glow plug working to assist in starting an internal combustion engine, and more particularly to such an apparatus equipped with a microcomputer designed to control an operation of a glow plug for avoiding overheating thereof.
2. Background Art
Usually, typical diesel automotive vehicles are equipped with a glow plug working to assist in heating inside a cylinder of a diesel engine up to an ignition temperature of an air-fuel mixture, especially when the outside air temperature is low, and the engine is cold.
In recent years, in order to meet requirements to improve the startability of the engine, engine control systems have been employed which are designed to apply a dc voltage to the glow plug to heat the glow plug to a target temperature quickly. Rough control of an on-duration for which the dc voltage is applied to the glow plug will, thus, result in increase in physical load on the glow plug. For instance, too long the on-duration of the glow plug will result in overheating of the glow plug, which may lead to wire breakage in the glow plug.
In recent years, for the purpose of increasing the accuracy in controlling the energization of the glow plug, glow plug energization control systems have been in widespread use which are equipped with a microcomputer designed to control power supply to the glow plug as well as other controls for the engine. When a key switch of the automotive vehicle is turned on, the electric power is supplied to the microcomputer. The microcomputer applies an effective voltage of 11V to the glow plug in the form of a PWM signal for a given initial energiation duration and then also applies an effective voltage of 7V to keep the glow plug at a target temperature of, for example, 900° C., for a given period of time (will also be referred to as a temperature-holding time below).
The above glow plug energization control systems, however, encounter a drawback in the following situation.
When the key switch is turned off by an operator in the course of energization of the glow plug and then turned on again immediately to reenergize the glow plug, the glow plug starts to be heated from a relatively high temperature. When the glow plug continues to be energized over the initial energization duration and the temperature-holding time, it may cause the glow plug to overheat.
In order to avoid the above problem, Japanese Patent First Publication No. 2004-108189 teaches a glow plug energization control system designed to keep the microcomputer activated after the key switch is turned off until the temperature of the glow plug decreases to a given lower level to calculate an optimum amount of energization of the glow plug which will not result in the overheating of the glow plug when the key switch is turned on immediately. This system, however, requires an electrical circuit which measures the voltage and current given to the glow plug to determine the optimum amount of energization, thus resulting in complexity and increase in production cost of the system.